FIG. 28 is a perspective view showing a throwaway insert 101 (hereafter abbreviated as “insert 101”). FIG. 29 is a side view showing the insert 101. The insert 101 is made attachable to a holder having substantially a cylindrical shape. The holder fitted with the insert 101 is used as a milling tool for milling a workpiece. For example, the insert 101 is attached to a milling tool such as a face mill and an end mill, and especially a milling tool having a long cutting-edge length to which are attached a multiplicity of cutting edges.
There has heretofore been known the insert 101 as shown in FIG. 28. In this construction, a rake face 104 which connects to a main cutting edge 103 has a positive rake angle, and a relief face 105 and the main cutting edge 103 are divided into pieces by grooves 106 (for example, refer to Japanese Unexamined Patent Publication JP-A 7-299636 (1995)). Such an insert 101 is used mainly for a heavy cutting-adaptable milling tool. In a heavy cutting, large amounts of chips are evacuated at a single cutting operation. Moreover, the depth of cut and the feed are so high that the process needs to be carried out under strict cutting conditions, and thus the milling tool receives application of a great cutting resistance from a workpiece. In the insert 101, since the main cutting edge 103 is divided into a plurality of main cutting edge divisions 103a by the grooves 106, it follows that chips produced in the cutting are broken into pieces. This helps reduce cutting resistance and thus improve the biteability of the insert on the workpiece, wherefore chattering can be suppressed during the cutting.
FIG. 30 is a perspective view showing a milling tool 112 fitted with the prior art insert 101 shown in FIG. 28. FIG. 31 is a schematic diagram of a cross section of part of the milling tool 112 to which is attached the insert 101, illustrating how a chip 116 is produced. The chip 116 originating from the workpiece in the course of heavy cutting is large in thickness, namely cross-sectional area, and has high rigidity and is thus less prone to deformation. Therefore, the chip 116 produced by the action of the main cutting edge 103 of the milling tool 112 runs on the rake face 104 centrally of a holder 111 and then bumps against a holder wall 114 of a chip pocket 113. Next, the chip 116 is evacuated out of the chip pocket 113 through the holder wall 114 acting as a guide. In this case, however, the chip 116 comes into collision with the holder wall 114, which gives rise to a problem of friction between the holder wall 114 and the chip 116. If the workpiece is subjected to the cutting repeatedly in this situation, as the process proceeds, so the holder wall 114 of the chip pocket 113 receives wear and is eventually chipped off. The chips 116 find their way into the chipped-off region of the wall, thus causing deterioration in chip evacuation.
This problem is causative of chipping-off of the insert 101 and reduced rigidity of the holder 111, which leads to poor processing accuracy. In a heavy cutting in particular, the thicker is the chip 116, the more likely it is that the holder wall is subjected to severe abrasion. After all, quite inconveniently, the damaged holder 111 will have to be replaced with the new one early.